Hydrogen sulfide removal and sulfur recovery

ABSTRACT

The present invention is directed to a process for removing H 2  S from a gaseous stream, preferably a natural gas stream, which comprises forming a buffered aqueous H 2  S stream containing thiosulfate by contacting the gaseous stream with water containing a buffering agent and thiosulfate to remove the H 2  S from the other gases in the gaseous stream; introducing the buffered aqueous H 2  S stream into a reduction section under conditions wherein elemental sulphur is produced and removing an effluent stream from the reduction section; introducing the reduction section effluent into an oxidation section and contacting the effluent with air to react the oxygen in the air with the H 2  S in the effluent stream to produce predominantly thiosulfate, the volume of liquid fed to the oxidation section being in excess of the volume of gas fed, and removing an effluent stream from the oxidation section; separating the effluent stream from the oxidation section into a gas stream and a liquid stream; and treating separately the gas stream and the liquid stream to remove H 2  S from each stream and additionally to recover elemental sulfur from the liquid stream.

FIELD OF THE INVENTION

The present invention is directed to a process for removing hydrogensulfide (H₂ S) from a gaseous stream and recovering elemental sulfur.More specifically, the process of the present invention is particularlysuited to treat a gaseous stream which contains in excess of fivepercent of H₂ S and is available at high pressures. For example, naturalgas in a sulfur environment may contain H₂ S at levels from one percentto ninety plus percent and may be at pressures of 55 atmospheres gauge(atg) or more. The H₂ S may be removed by the present invention to makethe treated gas suitable for commercial use and, in addition, recoverelemental sulfur.

BACKGROUND OF THE INVENTION

Hydrogen sulfide is often present in gas streams as a contaminant whichprevents the use of the gas for domestic, commercial or industrialpurposes. This problem is particularly severe in sour natural gas, whichis often produced with H₂ S concentrations from one percent to as highas ninety percent. Over the years, many desulfurization processes havebeen developed in attempts to produce gas streams substantially free ofhydrogen sulfide.

The commercial process most used in the recovery of hydrogen sulfidefrom an acid gas or sour gas stream and the production of elementalsulfur is the Claus process. The gas stream containing the acid gas isusually treated by solvent extraction or washing out the acid gases withany number of suitable solvents. The extraction or washing step producesa clean, treated gas stream and an acid gas stream. In the Clausprocess, the acid gas stream, mainly H₂ S, and a controlledstoichiometric quantity of air are fed into a reaction furnace, whereone-third of the H₂ S is burned to SO₂. The H₂ S and SO₂ react to formelemental sulfur thermally in the furnace. Also, elemental sulfur iscatalytically formed in the reactors which follow the sulfur furnaceaccording to the Clause reaction. One such commercial process isdisclosed in Hydrocarbon Processing, April 1982, p. 109.

Another commercial process for the removal of hydrogen sulfide and thepartial removal of organic sulfur compounds from natural and industrialgases is the Stretford process. The sour natural or industrial gas iscounter-currently washed with an aqueous solution containing sodiumcarbonate, sodium vanadate and anthraquinone disulfonic acid (ADA). Thehydrogen sulfide dissolves in the alkaline solution and is removed toany desired level. The hydrosulfide formed reacts with the 5-valentstate vanadium and is oxidized to elemental sulfur. The aqueous solutionfor extracting the sour gases is regenerated by air blowing, and thereduced vanadium is restored to the 5-valent state through a mechanisminvolving oxygen transfer via the anthraquinone disulfonic acid. Aspecific example of this process is set forth in Hydrocarbon Processing,April 1982, p. 112.

Still another process for the conversion of H₂ S to elemental sulfur isthe LO-CAT process. This process utilizes a dilute solution of iron heldin solution by organic chelating agents. The aqueous solution containingthe chelated iron serves as both a catalyst in the overall reaction ofH₂ S with oxygen and takes part in the reactions by transfer ofelectrons. A more specific description of the process is set forth inHydrocarbon Processing, April 1985, pp. 70 and 71.

U.S. Pat. No. 4,487,753 discloses a process for producing liquidelemental sulfur from a CO₂ -rich gaseous stream containing H₂ S. Thegas is contacted with at least a stoichiometric amount of gaseous oxygenin the presence of liquid water with a fixed bed comprising a catalystselected from the group consisting of a transition metal phthalocyaninecompound dispersed on a support at a specified pH and temperature. Thepatent discloses a preferred support as activated carbon.

U.S. Pat. No. 4,579,727, issued on the application which included thepresent inventor, discloses a process for recovering elemental sulfurfrom a hydrogen sulfide containing gas stream by reacting the hydrogensulfide in the gas stream with a buffered aqueous solution enriched inthiosulfate ions at an initial pH between about 4.5 and 6.5 for aresidence time sufficient to react a portion of the hydrogen sulfide toelemental sulfur. The elemental sulfur is them removed and the solutionnow lean in thiosulfate ions is regenerated by the oxidation of theremaining hydrogen sulfide in the gas stream to deplete the hydrogensulfide from the gas stream and to regenerate the liquid solution forrecycling to the reduction zone.

In many respects, the method of U.S. Pat. No. 4,579,727 has advantagesfor the removal of H₂ S from gas streams and production of sulfurtherefrom. However, this method is carried out in the presence of theentire volume of the gas being treated, requiring the use of largereaction vessels. This method requires the continuous addition ofcaustic because the process produces a substantial proportion of lowvalue sulfates and the loss of the production of sulfur. Furthermore,with this process, the sulfur product may be contaminated with H₂ S.Finally, if a nitrogen contamination of the gas stream is not allowed,it is necessary to use pure oxygen in the oxidation reaction.

SUMMARY OF THE INVENTION

The present invention is directed to a process for removing H₂ S from agaseous stream, such as a natural gas stream, which comprises contactingthe gaseous stream with water containing a buffering agent andthiosulfate ions to remove the H₂ S from the other gases in the gaseousstream; introducing the buffered aqueous stream containing H₂ S into areduction section under conditions wherein elemental sulphur is producedby reaction of H₂ S and thiosulfate ions; introducing the reductionsection effluent into an oxidation section and contacting it with air toreact the oxygen in the air with most of the remaining H₂ S in theeffluent stream in liquid phase to regenerate thiosulfate ions;separating the effluent stream from the oxidation section into a gasstream and a liquid stream; and treating separately the gas stream andthe liquid stream to remove residual H₂ S from each stream andadditionally to recover elemental sulfur from the liquid stream.

In a preferred embodiment of the invention, an absorber is used toabsorb the H₂ S from the gas stream and produce a gas streamsubstantially free of H₂ S which, in addition, leads to very high H₂ Sconcentration in the liquid, especially when the absorption is carriedout at high pressure and low temperature. This has the consequence ofreduction of the reduction vessel size. This is further helped by theabsence of any unnecessary gas associated with the aqueous streamcontaining the H₂ S.

In the oxidation section of the process of the present invention, bykeeping a discontinuous gaseous phase in a continuous liquid phase inthe oxidation reactor, such as by using in-line mixers, a very smalloxidation reactor results. By leaving a preferred, significant butsmall, amount of unconverted H₂ S, sulfate by-product formation isconsiderably reduced. This further reduces the amount of alkali to beadded, necessary for maintaining buffer activity within the desired pHrange.

In the process of the present invention, effluent from the oxidationsection may be recycled either to the reduction section or the oxidationsection or to both. The purpose of the recycle is to maintain thedesired level of buffering, to control the temperature, and to insurethe maintenance of a continuous liquid phase in the oxidation section.

Finally, the separate work-up of the gas and liquid phase-out of theoxidation section allows preferred conditions for each of these steps,further reducing reactor sizes and sulfate formation.

As to the sulfur produced, separation of sulfur after practically all H₂S has been removed, allows attainment of high quality sulfur,practically free of nocuous H₂ S.

Separation of the H₂ S and maintaining the liquid away and separate fromthe original gas stream, permits the use of air as a source of oxygen,further reducing the cost of the process.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more readily apparent from the appendeddrawings;

FIG. 1 diagramatically illustrates the process of the present invention.

DESCRIPTION OF THE INVENTION

According to the present invention, sour natural gas is contactedcountercurrently in an absorber by an aqueous solution containingthiosulfate ions and a buffering agent. The aqueous solution absorbs thehydrogen sulfide (H₂ S) present in the sour gas and the lean gas flowsto a pipeline or the like for wellknown uses. The pressure of theaqueous effluent of the absorber containing the absorbed H₂ S may bereduced in a flash drum and the vaporized portion, comprising primarilymethane and inorganics, is returned to the absorber. The liquid exitingthe flash drum, containing the aqueous solution with the thiosulfateions, the buffering agent and the absorbed H₂ S, is heated by indirectand direct means, hereinafter described, to a temperature of about 120°to 160° C., for flow through a reduction reactor, wherein thethiosulfate ions and a portion of the H₂ S react to form elementalsulfur and water by the following reaction:

    2H.sub.2 S+2H.sup.+ +S.sub.2 O.sub.3.sup.-- →4S+3H.sub.2 O (1)

The aqueoous effluent of the reduction reactor, lean in thiosulfate ionsand containing the buffering agent, unreacted H₂ S, elemental sulfur,flows to an oxidation reactor wherein most, but not all, of theremaining H₂ S is oxidized in the presence of oxygen regeneratingthiosulfate ions by the following reaction:

    2H.sub.2 S+2O.sub.2 →S.sub.2 O.sub.3.sup.-- +2H.sup.+ +H.sub.2 O (2)

Some H₂ S is left unreacted so that reaction (2) above dominates andsuppresses by-product sulfate formation according to the followingreaction:

    S.sub.2 O.sub.3.sup.-- +2O.sub.2 +H.sub.2 O →2 SO.sub.4.sup.-- +2H.sup.+                                                 (3)

The effluent from the oxidation reactor flows to a separator drumwherein the vapor is separated from the liquid and flows to a tail gasunit for further treatment hereinafter described. As regards the liquidphase of the oxidation reactor effluent, a portion thereof can berecycled to the reduction reactor, another portion can be recycled tothe oxidation reactor and the remaining portion is further treated toremove remaining traces of H₂ S and thereafter taken to a separatorwhere the elemental sulfur is removed. After removal of the sulfur,water can be removed by vacuum flash. Following such removal, theremaining buffered aqueous liquid is recycled to the front end of theprocess and, more particularly, to the absorber for use as the absorbentfor the removal of H₂ S from the sour natural gas. Before entering theabsorber, the recycled aqueous stream exchanges heat with the reductionreactor feed to preheat such feed. Furthermore, the recycled aqueousstream is upgraded by purging a small amount thereof to prevent thebuildup of impurities, and by adding, if desired, makeup water andbuffering agent lost in the cycle.

Referring to FIG. 1, which is a specific embodiment illustrating thetreatment of natural gas, the present invention can best be understoodas having four essential processing units: a gas treating unit 10 whosebasic function is to absorb the H₂ S from the other gases such as themethane, other hydrocarbons and other gases in natural gas in a bufferedaqueous stream rich in thiosulfate ions; an H₂ S conversion unit 40characterized by a reduction section wherein the H₂ S reacts with thethiosulfate ions to produce elemental sulfur and an oxidation sectionwherein most of the remaining H₂ S is reacted with oxygen from air toproduce thiosulfate ions; a liquid treating stream unit 70 whichprovides for the removal of any residual H₂ S in the liquid streameffluent from the oxidation section and the recovery of elementalsulfur; and a gas treating unit 110 which provides for treatment and/orremoval of H₂ S in the gas stream or tail gas from the oxidationsection.

In the gas treating unit 10, an acid gas stream 11 is introduced into anabsorber 13. The gas in gas stream 11 is contacted with a bufferedaqueous stream rich in thiosulfate ions introduced by line 15 intoabsorber 13. The absorber 13 may be a conventional gas-liquid absorberhaving a plurality of trays sufficient in number to remove essentiallyall of the H₂ S from the gas stream, leaving a practically H₂ S-free gasstream which exits by line 17. The absorber 13 may also be a packed bedor other device to remove the H₂ S.

The buffered aqueous stream containing absorbed H₂ S is removed by line19 and introduced through a pressure-reducing valve 20 into a flash tank21. Due to the reduction in pressure in flash tank 21, methane and othergases are released and are removed by line 23. These gases arecompressed by a compressor 25 and reintroduced into the bottom ofabsorber 13. The liquid is removed from the bottom of flash tank 21 byline 27 and may be heated in heat exchanger 29. Valves 31 and 33 areused to control the amount of heat exchange by the heat exchanger 29.The use of the heat exchanger 29, its size and the amount of heatexchanged are primarily a function of the total amount of H₂ S beingtreated in the system, as will be explained in more detail hereinafter.

The heated buffered aqueous stream is introduced by line 35 to reactor47 in the reduction section 41. Reactor 47 is preferably a fixed-bedreactor or a packed tower. The reactor is designed in a mannerwell-known to those skilled in the art to provide sufficient residencetime for the predominant reaction (2) above to occur. The conditions inthe reduction section 41 are preferably maintained at about 120° C. toabout 160° C. and at a pressure of between 10 and 40 atmospheres gauge.Under these conditions, the predominant reaction is between the absorbedH₂ S and thiosulfate ions to produce elemental sulfur and water. Also,under these conditions, the elemental sulfur exists essentially as aliquid in the aqueous stream. The effluent buffered aqueous stream fromthe reduction section 41, removed by line 49, contains the elementalsulfur, unreacted H₂ S and is now lean in thiosulfate ions. The amountof H₂ S in the effluent stream from the reduction section 41 when steadystate conditions are reached is approximately one-half of the H₂ Sintroduced into the system.

The effluent from the reduction section 41 passes by line 49 to in-linemixer 51 in the oxidation section 43. An oxygen-containing gas,preferably air, is also fed by line 53 to the in-line mixer 51. Mixer 51is preferably of the type that causes a large gas-liquid interface and,more specifically, a discontinuous gaseous phase in a continuous liquidphase for the co-current flowing phases. For instance, in-line mixer 51may be of the Komax or the Koch-sulzer type which provide very efficientand effective mixing of the gas and liquid. A suitable in-line mixertype is disclosed in "Gas/Liquid Mass Transfer with Static Mixing Units"by F. Grosz-Roll, J. Battig and F. Moser, published at the FourthEuropean Conference on Mixing, Apr. 27-29, 1982, incorporated herein byreference. This type of mixer has the capability of producing a verylarge number of very small, uniform-size bubbles of gas in thecontinuous liquid phase. The volume of liquid in the mixer is greaterthan the volume of gas, preferably by a ratio of from about 1.05:1 to3.0:1 or greater, so as to insure the maintenance of a discontinuousgaseous phase in a continuous liquid phase. This results in very fastmass transfer of the oxygen to react with the H₂ S according to reaction(2) above. The reaction, being exothermic, increases the temperature toan effluent temperature generally in the range of 135° C. to 170° C.

The amount of air introduced to the mixer 51 is a function of the amountof H₂ S present in the aqueous stream entering the mixer. According tothis invention, it is not desired to react all of the H₂ S, but to haveH₂ S gas in the outlet 55 of the in-line mixer 51 at a partial pressure,measured at 150° C., of at least 0.005 atmosphere absolute (ata) and,preferably, a partial pressure of 0.07 ata. thus, the amount of oxygenfed to the mixer is limited to less than would be fed to achieveessentially complete oxidation of H₂ S. It has been found, according tothe present invention, that the presence of at least 0.005 ata partialpressure H₂ S minimizes the production of undesirable sulfates, becausewhen less H₂ S is present, there is substantial oxidation of thethiosulfates ions, according to reaction (3) above.

The effluent stream from the oxidation section 43, which is the hottestpoint in the system with a temperature usually in the range of about135° C. to 170° C., is removed from the outlet of the in-line mixer 51by line 55 for introduction into a gas-liquid separator section 45,where it is separated into a gas stream and an aqueous stream. Theaqueous stream is treated in liquid treating unit 70 and the gas streamis treated in tail gas unit 110, as hereinafter described.

A portion of the liquid phase is removed from the gas-liquid separatorsection 45 as a hot liquid stream by line 57 and is utilized for tworecycles.

The first recycle is accomplished by passing a portion of this streamthrough valve 59 and pump 61 to line 63 for introduction into theoxidation section 43 or more specifically into the in-line mixer 51.This recycle enables the maintenance of the volume of liquid fed (V_(L))to the in-line mixer 51 to be in excess of the gas fed (V_(G)). Apreferred range of V_(L) /V_(G) is 1.05 to 3. Hence, the amount ofrecycle is well within the skill of an engineer, recognizing that theamount of gas introduced in the effluent stream from the reductionsection 41 while still maintaining at least the desired partial pressureof H₂ S, as previously described, in the effluent from the oxidationsection.

The second recycle is provided by passing the stream from line 57through valve 65 and pump 67 for reintroduction by line 69 into thereduction section 41. This recycle has two purposes; first, to regulatethe buffering capacity so that the pH in the reduction section 41 may beproperly maintained and second to increase the temperature of the mixedstreams into the reduction section to the desired temperature range.

According to the present invention both the gas stream or tail gas andthe liquid stream from the separator 45 may be separately treated forremoval of H₂ S to levels for disposal or recycle. The liquid stream isremoved from the gas-liquid separator section 45 by line 71. Dependingupon the amount of H₂ S which remains in the aqueous phase, a reductionvessel 73 may be utilized to provide sufficient residence time to permitfurther reaction of the H₂ S and thiosulfate to form additionalelemental sulfur. Alternatively, if the H₂ S concentration is small,reduction vessel 73 may be by-passed. The by-pass stream or the effluentfrom the reduction vessel 73 is then passed through steam stripper 75.Steam is added by line 77 at the bottom of steam stripper 75 forcountercurrent flow with the aqueous stream to remove any residual H₂ S,producing a exit stream 79 which, after compression, if required, may becombined with the aqueous H₂ S stream introduced by line 35 to thereduction section 41. The essentially H₂ S-free aqueous stream resultingfrom the steam stripping and containing the liquid sulfur is introducedby line 81 to a settler 83. In the settler 83, the liquid elementalsulphur is permitted to settle and is removed by line 85. Thesulfur-free liquid from settler 83 is removed by line 87 and passedthrough a pressure reduction valve (not shown) to a flash vessel 89 atlower pressure. A gas phase is removed, which is the water of reactionor any water added to the system, by line 92 whereas the liquid phase isremoved by line 93, the liquid stream containing thiosulfate andbuffering agent. This liquid stream in line 93 is then passed throughheat exchanger 29 for heating the incoming buffered aqueous H₂ S stream.

After the liquid stream is passed through heat exchanger 29, anyundesired heat in the liquid (for adjustment of the temperature of theliquid to absorber 13) is removed by passing through a heat exchanger 95to reduce the temperature to a preferred range of 10° C. to 60° C. andthen passed through line 97 to a pump 99 for recycle to line 15 ofabsorber 13. In line 97 is a line 101 for taking off a bleed, primarilynecessary for taking out the small amounts of sulfate which may beproduced. Make-up water is introduced by line 103 and buffering solutionintroduced by buffering solution unit 105. Therefore, the necessarybuffered aqueous stream containing the thiosulfate is ready forintroduction by line 15 to the absorber 13.

The tail gas unit 110 may employ any of the commercial tail gasprocesses used in association with the Claus sulphur recovery units. Forexample, the Bevins sulfur recovery or catalytic incineration or CBAprocess, each of which are described in Hydrocarbon Processing, April1982, pp. 114, 115 and 116, respectively. A preferred unit, however, isdisclosed in FIG. 1.

The gas stream from the gas-liquid separator unit 45 is removed by line111. The gas is introduced into a oxidizer unit 113. A portion of therecycled liquid stream from line 15 is also introduced by line 115 intothe oxidizer 113. Air or oxygen-enriched air is introduced by line 117to reactor with the hydrogen sulfide to produce thiosulfate. Because ofthe presence of thiosulfate, some elemental sulphur will also beproduced. A stream is removed from the oxidizer 113 by line 119 andintroduced into a gas-liquid separator 121. A portion of the liquid fromseparator 121 is cooled by a heat exchanger (not shown) and recycled byline 123 and pump 125 for reintroduction by line 127 to the oxidizerunit 113. In the recycle line 123 is a line 129 for purge of a portionof the liquid due to any build-up of any sulfates. The remaining portionof the liquid is removed by line 131 and may be combined or treatedseparately from the gas-liquid separator 45 for recovery of the sulphuras well as removal of any residual H₂ S. The gas removed by line 133 hasan H₂ S residual which meets specifications for disposal, afteroxidation to SO₂, either by flaring or other means.

The buffering agent used in the process of this invention may be any ofthe well-known buffering agent which may be utilized to maintain a pH onthe slightly acid side, such as between a pH of 4 and 7. As iswell-known to those skilled in the art, such systems as thecarbonate-bicarbonate or the phosphate buffering solutions may beutilized, and they are usually utilized in the form of the alkali metalsalts, preferably sodium or potassium. Preferred buffering agents arethe sodium carbonate and sodium bicarbonate. Preferably, the amount ofbuffering materials used is from about 0.5 to 2 gram moles per liter oftotal solution, to obtain a pH of the aqueous stream flowing into thereduction section between 4 and 7, preferably 4.5 to 5.5, for reactionof H₂ S with thiosulfate ions to produce sulfur.

The thiosulfate ions in the process will usually be in the form of thesalt of the alkali metal of the buffering agent. It will be appreciatedthat the thiosulfate ions are the result of the oxidation reaction, andthat actually a number of other sulfur anions result from this reaction,including polythiosulfates as well as monothiosulfates. The term"thiosulfate ion", as used herein, is therefore not intended to belimited to S₂ O₃ ⁻⁻, but includes all such ions which may be producedand are reactive with H₂ S.

It will also be appreciated that the amount of thiosulfate ions iscontrolled by the amount of oxygen introduced to the oxidation section,the amount of oxygen being based on the amount of H₂ S in the aqueousstream being fed to the oxidation section.

Referring to reactions (1) and (2) hereinabove, hydrogen ion consumptionincreases the pH, and in the oxidation reaction the pH is reduced again.The use of buffering materials aids in maintaining the pH within thedesired range; however, the process of the present invention provides ameans for maintaining the pH within the desired range in addition to theaddition of buffering materials. More specifically, this is accomplishedby the second recycle, through line 69, which provides a relativelylarge volume of buffered aqueous solution to the reduction section.According to this invention, sufficient recycle buffered solution isprovided to reduce the H₂ S concentration fed to the reduction unit tono more than about 0.4 gram moles per liter. This allows the productionof approximately 0.2 gram equivalent of hydrogen ions in the reductionsection and the removal of approximately 0.2 gram equivalent in theoxidation section, while still maintaining the desired pH range. Theamount of aqueous solution required to limit the H₂ S concentration to0.4 gram moles per liter is a molar ratio of H₂ O to H₂ S of 139:1 (1000gm/l/18=55.56 gram moles/1 H₂ O divided by 0.4 gram moles/2 H₂ S=139).However, ratios as low as 80:1 may be used with good results, as maymuch higher ratios, e.g., 200:1 or more.

In order to insure that the temperature in the reduction section is inthe desired range, preferably 120° C. to 160° C., it may be necessary toincrease the flow through the second recycle at some concentrations ofH₂ S.

A significant aspect of the present invention is that when the H₂ Slevel in the inlet stream is greater than about five percent (5%) thereis sufficient heat produced in the H₂ S conversion unit 40 for theoperation of the process. A temperature of 130° C. to 170° C. at theexit of the separator 45 is desired to maintain the molten sulfur at lowviscosity. Furthermore, it is desirable to maintain the temperature inthe reduction section in the range of from 120° C. to 160° C. The heatof reactoin in the oxidation section provides the heat necessary tomaintain these temperatures. At low H₂ S concentrations and when lowrecycle is required, less heat is generated. Hence, more heat isprovided by heat exchanger 29 to obtain the desired temperature in thereduction section. At high H₂ S concentrations and when more recycle isrequired, more heat is generated and less heat exchange is needed.

In some instances at very high H₂ S levels, for example in excess ofsixty percent (60%) H₂ S, in the inlet gas so much heat is developed inthe H₂ S conversion unit 40, that warm up from stream 35 to stream 71can not take this heat away, so that a cool stream is desired in saidunit 40. In those instances, a cooler or heat exchanger may be includedin the line after valve 59 or in line 57 (not shown). As noted, thepreferred position of the cooler is in the first recycle i.e., therecycle to the oxidation section 43. Cooling is preferably done with aliquid above 120° C. or under conditons not to form solid sulfur in thecooler or exchanger.

Under some circumstances, the recycle to the reduction section mayprovide sufficient liquid that the recycle to the oxidation section isnot needed to provide a continuous liquid phase in the mixer 51. On theother hand, at very low H₂ S levels, the recycle to the reductionsection may not be needed to maintain adequate buffering. Thus, thesystem has flexibility sufficient for efficient operation while treatinggases with a wide range of H₂ S concentrations.

The system and method of this invention are particularly well adaptedfor treatment of natural gas streams at high pressures because, at suchpressures, higher concentrations of H₂ S may be absorbed, allowingeconomy of scale in the equipment. However, the invention is also usefulfor treatment of lower pressure gases.

The present invention may also be further illustrated by the followingspecific examples:

EXAMPLE I

A feed of 5.6 million standard cubic feet per day (MMscfd) of naturalgas, containing nine percent of H₂ S, and at a pressure of 65 atg is fedto the process of the present invention. This feed is counter-currentlycontacted with an aqueous recycle stream containing 9500 pound moles perhour (MPH) of water, which is fed at about 40° C. and which ispractically free of H₂ S. The absorption is carried out in a multi-platebubble tower. The treated natural gas exits from the tower and containsless than 5 ppm H₂ S. The liquid stream, which now contains practicallyall the H₂ S, is removed from the bubble tower and introduced into aflash vessel at 40 atg. From the flash vessel is removed a gas which isrecompressed and fed back to the bottom of the absorption tower. Theliquid stream from the flash vessel contains about 55.4 MPH of H₂ S andabout 0.3 MPH of CH₄, together with small amounts of carbon dioxide,absorbed from the original natural gas stream. This liquid stream isheated in a heat exchanger, where heat exchange takes place against arecycled hot stream. The heated stream has an exit temperature of about135° C. This liquid is then fed to the H₂ S conversion unit whichcomprises first a reduction vessel at a pressure of about 15 atg,without any recycle required. In the reduction vessel, plug flow ismaintained by the presence of known internal structures. A suitablevolume for the reduction vessel is about 350 cubic feet (cf).

The liquid effluent stream from the reduction vessel containsapproximately 28 MPH of H₂ S in solution. This effluent stream is fed toan oxidation vessel where most of the H₂ S is oxidized by air. A totalof about 133.3 MPH of compressed air, which is about 28 MPH of O₂ andabout 105.3 MPH of N₂, is introduced into the oxidation vessel. Theoxidation reactor is a series of Koch-type SMV static mixer elements asillustrated by the paper referred to hereinabove. The volume of the airis about 9,500 cubic feet per hour (cf/hr). This is at a temperature of150° C. and 8 ata partial pressure. The total volume of liquid being fedis approximately 10,000 cf/hr of liquid, or about 35,000 MPH. To achievethis, a recycle of about 25,500 MPH of hot liquid from the gas-liquidseparator at the exit of the oxidation vessel is added. The oxidationvessel is about 1'-7" in diameter and may be 20° long filled within-line mixers to provide a gas dispersed in liquid, i.e. adiscontinuous gaseous phase in a continuous liquid phase. The pressureat the inlet of the oxidizer is approximately 14 atg.

From the oxidation vessel is removed an effluent stream which is fed toa gas-liquid separator. The separator separates a gas strem which has anH₂ S partial pressure of 0.08 ata and a liquid stream. The level of H₂ Sin the gas stream is accordingly about 1 MPH of H₂ S.

The gas from the gas-liquid separator is fed to a tail gas recovery unitwhich includes an after-oxidation reactor. A cooled liquid recyclestream contains about 5,000 MPH of water which is from the gas-liquidseparator following the after-oxidation reactor, is added to theafter-oxidation reactor. Also added to the after-oxidation reactor, areabout 10 MPH of air (2.1 MPH of O₂ and 7.9 MPH of N₂)and a cold recyclecontaining about 350 MPH of water (which will be described hereinafterbelow). The oxidatio in the tail gas unit is carried out at atemperature of about 80° C. The effluent from the after-oxidationreactor is introduced into a gas-liquid separator. The gas from thegas-liquid separator of the tail gas unit has an H₂ S content which isbelow 250 PPM on a total dry gas basis. This level is suitable fordisposal or simple burning in any manner that will convert the verysmall amount of H₂ S into SO₂ which then can be disposed into theatmosphere. From the liquid stream from the gas-liquid separator in thetail gas unit, the aforementioned stream containing 5000 MPH of water isrecycled, and the rest is then combined with the liquid from thegas-liquid separator from the H₂ S conversion unit.

A liquid stream from the gas-liquid separator in the H₂ S conversionunit provides the aforementioned recycle, containing about 25,500 MPH ofwater to the oxidation vessel. The remaining liquid stream is thencombined with the liquid stream from the tail gas unit. These combinedliquid streams contain about 9,900 MPH of water and approximately 0.4MPH of H₂ S. The combined liquid stream is fed to an after-reductor,which is about 900 cf, wherein the H₂ S is reduced to an amount of about0.00135 MPH, sufficient for ultimate recycle to the cold absorber.Before the recycle, however, the liquid sulphur is separated in aliquid-liquid separator. The hot aqueous stream is flashed to removewater of reaction and is fed in exchange with the effluent from the coldabsorber, maximizing the use of the heat within the system. After thisheat exchange, the liquid is cooled to about 38° C. and recycled to thecold absorber. A part of this stream, containing 350 MPH of water, isthat used in the tail gas recovery oxidation unit.

EXAMPLE II

A base feed of about 0.93 MMscfd of a natural gas stream containing 55%H₂ S, 2% carbon dioxide and 43% CH₄, is introduced to an absorber at apressure of 70 atg. The feed is counter-currently contacted in theabsorber with an aqueous recycle stream containing 1500 MPH of water.The recycle stream is fed into the absorber at about 40° C. and ispractically free of H₂ S but contains a buffer solution of sodiumcarbonate-sodium bicarbonate and thiosulfate. The exit gas from theabsorber, which is a multi-plate bubble tower, contains less than 5 PPMof H₂ S. The liquid removed from the absorber is passed through apressure reduction valve and flashed in a flash vessel at about 45 atg.The gas from the flash vessel is compressed and fed back into the bottomof the absorption tower. The liquid from the flash vessel then containsabout 56 MPH of H₂ S, 0.3 MPH of CH₄ and about 0.5 MPH of carbondioxide. Due to the heat of absorption, the temperature of the liquid isat about 47° C. The liquid is fed to a heat exchanger which allows somewarm up, if heat loss occurs within the system, for temperature controlof the reactor. A bypass of the heat exchanger is available sincesufficient heat is available to the system.

The liquid stream is then fed to a reduction vessel in the H₂ Sconversion unit. The H₂ S conversion unit consists of a reductionsection, an oxidation section, a gas-liquid separation section and tworecycles within the H₂ S conversion unit, each having a pump to recyclethe relatively gas-free liquid from the gas-liquid separation section atthe outlet of the oxidation section. The inlet liquid is introduced intothe reduction vessel together with 5.15 times its volume of a recyclestream. Since the recycle liquid stream is at about 150° C. when mixedwith the inlet liquid, the resulting liquid temperature is at about 132°C. The pressure in the reduction vessel is at about 20 atg. Packing ispresent in the reduction vessel to insure the desired plug flow andprevent undue amounts of back-mixing. The volume of the reduction vesselis about 300 cf which is sufficient to remove approximately 50% of theH₂ S by the reaction (1) hereinabove. In the reduction of the H₂ S,there is developed about 28 lb. equivalent of alkali. The buffer in thesolution is more than sufficient to counteract the effect of theproduction of this amount of alkali. From the foregoing, the streamcontains 1,500 MPH of water plug 5.15 times 1,500 MPH for a total of9,225 MPH of water. The amount of alkali in this amount of watertranslates to 0.17 g mol/1[28/(9225×18/1,000)]. Since a proper loadingof buffer will handle at least 0.2 g mol/l (gram mole per liter) with asmall resultant pH change, the recycle of 5.15 times is adequate formaintaining the proper pH within the reduction reactor. The molar ratioof H₂ O to H₂ S is thus 9225:56 or 165:1.

The liquid is then introduced to the oxidation unit which is operated ata pressure of 13 atg. The static mixing units of the Koch-type as setforth in Example I, provide a gas in liquid dispersion, i.e. adiscontinuous gaseous phase in a continuous liquid phase, whereby thegaseous oxygen reacts with the H₂ S in the liquid phase with thereaction being limited by the oxygen mass transfer in the gas phase. Toassure the gas in liquid dispersion, the volume of the liquid fed to theoxidation reactor is larger than the volume of gas fed to the reactor.Except for the start-up operation or during any swings in the amount ofH₂ S being introduced into the system, the amount of H₂ S being fed tothe oxidation reactor is about half that being introduced into thesystem or about 28 MPH, thus requiring that amount of oxygen. A totalamount of about 133.3 mol MPH of air is introduced (28 MPH of oxygen and105.3 MPH of nitrogen). The total volume of gas is about 9150 cf/hr. Toachieve an excess volume of liquid, more than 31,732 MPH of water isnecessary, and thus, a total flow of at least 22.22 times the fresh feedis needed. As set forth hereinabove 5.15 times the initial volumeintroduced into the H₂ S conversion unit or the reduction vessel hasbeen taken in the first recycle, and thus, the second recycle will be atratio of 16.07:1. The total liquid flow to the oxidation unit isapproximately 9610 cf/hr (22.22×1,500×18/62.43). A small excess in thesecond recycle is taken not only to assure that the volume of liquid fedto the oxidation reactor is in excess of the gas volume fed, but also toassure that the temperature is at or close to 150° C. The air flow tothe oxidation reactor is controlled to achieve a final H₂ S partialpressure of about 0.035 ata. At this level of H₂ S partial pressure, theoxidation beyond that of the H₂ S to thiosulfate is maintained so thatvery low sulfate make (less than 1% of hydrogen sulfide converted) isobtained in the oxidation reactor.

The liquid-gas mixture from the oxidation unit is then fed to ahorizontal gas-liquid separator. The two recycles aforementioned arefrom the liquid phase of the gas-liquid separator.

Separate treatment of the gas stream and liquid stream to remove H₂ Sand recover the sulphur takes place.

A stream is taken from the liquid phase in the gas-liquid separator. Thestream removed is substantially equivalent in volume to the volume ofthe liquid stream coming from the flash vessel after the absorber. Thisliquid stream contains the sulphur product in liquid form. When the H₂ Sgas phase has an outlet pressure of about 0.035 ata, the liquid phasecontains about 0.03 MPH of H₂ S, which must be removed prior to thisstream being recycled as the recycle stream introduced to the absorberto absorb H₂ S down to the 5 ppm level. The liquid stream accordingly isstripped in a countercurrent stream of about 40 MPH of steam at about150° C. The gaseous product containing the H₂ S and steam is cooled andrecycled as a cold liquid containing the H₂ S into the H₂ S converterunit at the reduction vessel inlet. After removal of the dissolved H₂ S,the liquid stream is passed to a horizontal settler to separate theliquid sulphur from the remainder of the hot liquid. Removed from thesettler is a sulphur product at a rate of about 55.5 MPH, taken asmonomolecular sulfur. The hot aqueous stream from the settler is reducedin pressure to permit evaportion of water, primarily to remove the waterof reaction to maintain the water balance within the system. The liquidphase, still being hot, may be passed in heat exchange with the coldfeed and, if necessary, cooled further before recycle to the absorber. Apump is used to introduce this recycle stream to the absorber at thepressure necessary to contact the higher pressure gas being washed withthis aqueous stream.

The gas stream from the gas-liquid separator is treated in similarfashion as described in Example I.

EXAMPLE III

This example is to illustrate an embodiment wherein the concentration ofH₂ S in a well is very high and the concentration of natural sulphurpresent in the well is so great that it is necessary to introduce a hotaqueous stream into the bottom of the well to avoid deposition ofsulphur and clogging of the well.

Injected into a well is an aqueous hot recycle stream containing 5,157MPH of water to absorb the major amount of H₂ S present in the well. Thewell produces at 500 atg. about 2 MMscfd of gas containing 90% H₂ S, 9%CH₄, 1% carbon dioxide and also about 3,000 lbs. of sulphur per MMcf ofgas. From the well, an effluent which contains about 176 MPH of H₂ S insolution in the aqueous stream is removed from the well. The temperatureof the liquid stream is at about 138° C. and the sulphur is in the formof a liquid dispersion. When the recycle stream used as the hot watersource contains thiosulfate, some of the H₂ S will react with thethiosulfate in the aqueous solution to produce more liquid sulphur.

From the well, exits also a gas phase which contains about 18 MPH ofCH₄, 1 MPH of CO₂ and about 22 MPH of H₂ S. This gas phase is separatedfrom the liquid phase and the gas phase is washed countercurrently atabout 70 atg with a recycle stream, it being the same source as thestream being placed in the well, containing 1,000 MPH of water at about40° C. The liquid from this absorption is combined with the hot liquidrecovered from the well. The combined streams are reduced in pressure toabout 25 atg and introduced into a reduction vessel of about 400 eftogether with a hot recycle stream from the H₂ S conversion unitcontaining 23,543 MPH of water. Since the total amount of H₂ S in thecombined liquid streams is 198 MPH, the molar ratio of total H₂ O to H₂S fed to the reduction vessel is 29,700:198 or about 150:1.

From the reduction vessel is taken a liquid stream which is fed to theoxidation unit. The oxidation reactor is a Koch-type in-line mixed asset forth in Example I. Also introduced to the oxidation unit is apractically gas-free recycle liquid from a gas-liquid separator at theexit of the oxidation unit. This recycled liquid contains approximately110,000 MPH of water. The temperature of this stream has been reduced byheat exchange against a stream of about 118° C. to reduce thetemperature of the recycle stream to approximately 144° C. Since thereaction in the oxidation unit is exothermic, considerable amounts ofheat are produced and with the amount of H₂ S being at very high levels,the excess heat must be reduced to take the heat of reaction out of theoxidation unit. In addition, compressed air containing approximately 100MPH of oxygen and 376 MPH of nitrogen is introduced to the oxidationunit. A suitable oxidation unit has a diameter of 3'-0" and a length ofabout 20', and is filled with in-lin mixers of the Koch-type. Thepressure within the oxidation unit is about 14 atg which results in agaseous volume at those conditions of about 30,000 cf/hr. A total liquidvolume feed is about 40,000 cf/hr, thus insuring a gas in liquiddispersion within the oxidation unit. The outlet H₂ S partial pressureis 0.08 ata.

At the exit of the oxidation unit is a gas-liquid separator. From theseparator is removed a liquid stream which is treated to remove residualH₂ S and recover the liquid sulphur. The stream contains about 0.2 MPHof H₂ S. To treat this stream, it is fed to a 700 cf plug flowafter-reducer vessel wherein the liquid leaves containing about lessthan 0.001 MPH of H₂ S or 0.1 part per million of H₂ S. The sulphur isrecovered in a separator vessel and the hot liquid flashed to remove thewater of reaction and the liquid is then recycled.

The gas from the gas-liquid separator may be treated by any of the usualtail gas processes for H₂ S. However, a suitable method is to combinethe gas which has a partial pressure of H₂ S of about 0.03 ata with aliquid recycle which contains 560 MPH of water and treat in similarfashion as described in Example 1.

Besides the advantages which have been described in the generaldescription of the present invention and illustrated by the examples, itshould be recognized that:

1 - sulfur is recovered which is essentially H₂ S-free;

2 - where the H₂ S can be separated from the remaining gses in the sourgas, the H₂ S is separated before reaction with the thiosulfate oroxygen containing gas to reduce the total volume of material pasingthrough the system;

3 - the size of the reduction reactor is minimized by maximizing the H₂S concentration in the aqueous phase introduced into the reductionreactor;

4 - the size of the oxidizer is minimized by the use of in-line mixersby causing a large gas-liquid interface;

5 - the economics of air rather than pure oxygen are fully utilized; and

6 - the production of sulfates is minimized by conversion of H₂ S withoxygen in the oxygen-containing gas but only to a level where the gasphase and the liquid phase can be separately and easily treated toremove the H₂ S to disposal or recycle levels.

It is understood that while the process of the present invention hasbeen illustrated by the removal of hydrogen sulfide from a gaseousstream, that the process is also directed to the recovery of elementalsulfur from various sources. In cleaning a gaseous stream, for example,a hydrocarbon stream such as natural gas, the treated gas is madesuitable for commercial use. In this regard, when treating natural gasand an absorber is used, the carbon dioxide (CO₂) present may also beremoved from the natural gas since most of the CO₂ is also removed fromthe natural gas in the absorber. The process of the present invention isalso suitable for treating a hydrogen gas stream such as is produced indesulfurization processes used in a refinery. In these instances, thegas streams may be cleaned or treated by the removal of H₂ S to enhancethe treated gas streams. The present invention may be used on treating agas without the use of an absorber, such as treating essentially acarbon dioxide stream. Furthermore, the process is also useful toprimarily recover the sulfur from a gaseous stream and the remaininggases may be considered of little interest since the sulfur is theprimary product.

Although the invention is described with respect to specific embodimentsand modifications, the details hereof are not to be construed aslimitations except to the extent indicated in the following claims.

I claim:
 1. A process for recovering elemental sulfur from a gaseousstream containing H₂ S which comprises:contacting said gaseous streamwith a buffered aqueous stream containing thiosulfate ions to form abuffered aqueous H₂ S stream containing thiosulfate ions and a productgas stream substantially free of H₂ S which is not further contactedwith thiosulfate ions; introducing said buffered aqueous H₂ S streaminto a reduction sectionunder conditions whereby elemental sulfur isproduced, and removing the aqueous effluent stream from said reductionsection; contacting said reduction section aqueous effluent in anoxidation section with oxygen to react H₂ S in said effluent tothiosulfate ions; and recovering elemental sulfur from the resultingstream from the oxidation section.
 2. process according to claim 1,including maintaining a continuous liquid phase in the oxidationsection. PG,29
 3. A process according to claim 2 wherein said continuousliquid phase is maintained by a recycle of a portion of the effluentfrom the oxidation section.
 4. A process according to claim 1 includingrecycling a portion of the effluent from the oxidation section to saidreduction section.
 5. A process according to claim 1 which furtherincludes separating the effluent from the oxidation reaction into gasand liquid streams, and separately treating the gas and liquid streamsto remove H₂ S.
 6. A process according to claim 5 wherein the separatedliquid stream is treated by passing it through a reduction zone underconditions for production of elemental sulfur and removal of H₂ S byreaction of the H₂ S and thiosulfate ions.
 7. A process according toclaim 6 wherein the separated liquid stream is further treated by steamstripping.
 8. A process according to claim 5 wherein the separatedliquid stream is treated by steam stripping.
 9. A process according toclaim 5 wherein the separated gas stream is treated with air to reactthe residual H₂ S at lower temperatures than used in the oxidationsection.
 10. A process according to claim 4 wherein the molar ratio ofH₂ O to H₂ S introduced to the reduction section is in excess of 80:1.11. A process according to any of claims 1, 2, 3, 4 and 10 wherein theoxidation reaction is with oxygen in air.
 12. A process for removing H₂S from a gaseous stream which comprises:contacting said gaseous streamwith a buffered aqueous stream containing thiosulfate ions to form abuffered aqueous H₂ S stream and an essentially H₂ S-free gas streamwhich is not further contacted with thiosulfate ions; introducing saidbuffered aqueous H₂ S stream into a reduction section under conditionswhereby elemental sulfur is produced and unreacted H₂ S remains, andremoving an aqueous effluent stream containing said unreacted H₂ S fromsaid reduction section; introducing said reduction section effluent intoan oxidation section, and contacting said effluent with air to reactless than all of the H₂ S in said effluent to thiosulfate ions;separating gas from liquid in the effluent stream from said oxidationsection; treating the separated gas stream to remove H₂ S from said gasstream; treating the separated liquid stream to remove H₂ S from saidliquid stream; and recovering elemental sulfur from the liquid stream.13. A process according to claim 12 which further includes recycling theliquid stream, after removal of the sulphur, to the H₂ S containinggaseous stream.
 14. A process according to claim 12 wherein further arecycle of a portion of said separated liquid is introduced to saidoxidation section.
 15. A process according to claim 12 wherein further arecycle of a portion of said separated liquid is introduced to saidreduction section.
 16. A process according to claim 12 wherein further arecycle of a portion of said separated liquid is introduced to saidoxidation section and a recycle of a second portion of said separatedliquid is introduced to said reduction section.
 17. A process accordingto claim 12 wherein the partial pressure of H₂ S in the effluent streamfrom said oxidation section is at least 0.005 ata.
 18. A processaccording to claim 12 wherein the partial pressure of H₂ S in theeffluent stream from said oxidation section is at least 0.07 ata.
 19. Aprocess according to claim 13 wherein the temperature of the effluentstream from said oxidation section is within the range of about 130° C.to 170° C.
 20. A process according to claim 13 wherein the temperatureof the liquid stream in said reduction section is maintained at about120° C. to about 160° C.
 21. A process for removing H₂ S from a gaseousstream which comprises:absorbing H₂ S from said gaseous stream in abuffered aqueous stream containing thiosulfate ions to form a bufferedaqueous H₂ S stream and an essentially H₂ S-free gas stream; heating thebuffered aqueous H₂ S stream to a temperature to enhance reaction of H₂S and thiosulfate ions to produce elemental sulfur; maintaining saidaqueous stream in a reduction section for a period sufficient to reactapproximately one-half of the absorbed H₂ S with thiosulfate ions;transferring the aqueous stream from the reduction section to anoxidation section; introducing air to said oxidation section to provideoxygen for reacting less than all the remaining H₂ S to form thiosulfateions; separating the effluent from the oxidation section into a gasstream and a liquid stream; recycling a portion of the liquid stream tomaintain the pH in the reduction and oxidation sections in the range of4.0 to 7.0; separately treating the remaining liquid stream and the gasstream to remove H₂ S; recovering elemental sulfur from the H₂ S-freeliquid stream; and returning the sulfur-free liquid stream to theabsorbing step.
 22. A process according to claim 21 wherein said recycleis sufficient to maintain a continuous liquid phase in said oxidationsection.
 23. A process according to either of claims 21 and 22 whereinsaid recycle is introduced to said reduction section.
 24. A processaccording to claim 23 and including a recycle to said oxidation section.25. A process according to claim 21 and including maintaining thetemperature of the separated liquid stream at a temperature of about130° C. to 170° C.
 26. A process according to claim 25 and includingmaintaining the temperature in the reduction zone at about 120° C. to160° C.
 27. A process according to either of claims 25 and 26 whereinthe partial pressure of H₂ S in the effluent from the oxidation zone isat least 0.005 atmospheres absolute.
 28. A process for converting H₂ Sto elemental sulfur and thiosulfate ions which comprises:introducing abuffered aqueous stream containing thiosulfate ions in the presence ofH₂ S into a reduction section under conditions whereby elemental sulfuris produced, and removing an aqueous effluent stream containingunreacted H₂ S and said elemental sulfur from said reduction section;introducing said aqueous reduction section effluent into an oxidationsection cocurrently with an oxygen containing gas, said aqueousreduction section effluent and said oxygen-containing gas being the onlyreactants introducted into said oxidation section, to react less thanall of the remaining H₂ S in said reduction section effluent tothiosulfate ions; separating gas from liquid in the effluent stream fromsaid oxidation section; recycling a portion of said separated liquidfrom said oxidation section to maintain the volume of liquid in saidoxidation section in excess of the volume of gas introduced to saidsection; treating the separated gas stream to remove H₂ S from said gasstream; and treating the remaining portion of said liquid stream toremove H₂ S and further recovering elemental sulfur from said liquidstream.
 29. A process according to claim 28 wherein said recycle isintroduced to said oxidation section.
 30. A process according to claim28 wherein said recycle is introduced to said reduction section.
 31. Aprocess according to claim 28 wherein one recycle is introduced to saidoxidation section and a second recycle is introduced to said reductionsection.
 32. A process according to claim 31 in which the recycle to thereduction section is sufficient to provide enough buffering that the pHwill not vary from the range of 4.0 to 7.0 during the reduction andoxidation reactions.
 33. A process according to claim 32 in which themolar ratio of H₂ O to H₂ S in the reduction section is at least about125:1.